**2. Body-site infection of clinically-important bacterial pathogens**

The clinical importance of different bacterial pathogens is widely recognized, and regular examination is required to provide an accurate diagnosis for specific kinds of infections. Some clinically important bacterial pathogens that cause health complications worldwide and occur in the mouth, skin, lungs, and gut are given below.

#### **2.1 Mouth**

The mouth, which serves as a pathway to the digestive system, offers a habitat for a diverse and abundant microbial population, and masses of these organisms and their products develop on the surfaces of the teeth and gums [14]. These growths, commonly referred to as plaque from the mouth and classified as biofilms, contribute to the development of cavities, which leads to tooth damage [15]. The microbial community that grows around teeth is extremely complicated. The microbiome of the mouth poses a threat to maintaining overall and dental health [16]. Therefore, dental

#### *Formation, Regulation, and Eradication of Bacterial Biofilm in Human Infection DOI: http://dx.doi.org/10.5772/intechopen.114177*

caries is a dietary-microbial disease that involves a cariogenic biofilm and continuous exposure to fermentable carbohydrates from dietary sources, such as sucrose, glucose, fructose, maltose, etc. [17]. Around 700 different types of bacteria in the mouth cavity have been detected using ribosomal identification techniques, among which *Streptococcus mutans* is one of the most common caries-causing bacterium [18]. It canmetabolize many types of carbohydrates, generating high-level acidity and dextran that facilitates the production of dental plaque. Many different bacterial species belonging to the genera *Streptococcus* and *Actinomyces* can be found in the plaque biofilms [19, 20]. For example, the two Gram-positive and anaerobic bacteria, *Streptococcus anginosus* and *Actinomyces naeslundii*, are commonly found in biofilms, while under healthy circumstances, Gram-negative bacteria such as *Aggregatibacter Actinomycetemcomitans*, *Campylobacter* spp*., Porphyromonas spp., Prevotella intermedia,* and *Treponema denticola* can also exist [21]. The investigations have shown that these Gram-negative bacteria might infect other parts of the body when hygiene in the mouth fails to be observed. For example, *staphylococci* (*staph*) and *streptococci* (strep are involved in endocarditis) [22]. In addition to this, other issues related to these biofilms include actinomycosis, dental root infections, and foul breath [23].

#### **2.2 Skin**

The human skin microbiome performs a significant role in both health and disease. The initial defense line of human body from pathogens is the skin, which protects and shields the body and provides a hostile environment for majority of bacteria [24]. Microbial biofilms are an extensively investigated mode of surface-associated growth that exhibits community-like characteristics. Furthermore, biofilms play an important role in numerous skin diseases. The usual microbiota of skin comprises a considerable number of Gram-positive bacteria, such as *Staphylococci* and *Micrococci*. Gram-positive bacteria are comparatively resistant to harsh conditions such as dryness and extreme osmosis pressures noticed in high salt or sugar mixtures [25]. The common causes of bacterial infections in skin are *Staphylococcus* and *Streptococcus* [26]. The bacteria *Streptococcus pyogenes* is responsible for a contagious bacterial skin infection that forms pustules and yellow, crusty sores. In certain instances, both *S. aureus* and *S. pyogenes* are present. Usually, the bacteria that cause infection penetrate through a small skin opening. Additionally, the infection has the potential to spread to neighboring body parts. However, the primary skin pathogens are coryneform bacteria like hemolytic *Streptococci* and *S. aureus* [27]. Normally, such bacteria penetrate the body from a wound in the skin, including bites from insects, etc. [28]. *In vitro,* single-species biofilms of skin microbiota, such as *S. aureus*, *S. epidermidis*, and *Propionibacterium acnes*, have been investigated [29]. Furthermore, important inter-species interactions with skin prokaryotes have been found, e.g., *S. epidermidis* inhibiting both *P. acnes* growth and *S. aureus* biofilms [29, 30].

#### **2.3 Lungs**

Lung infection is one of the most prominent health issues. Various systemically or respiratory problems start with lung infections. The region of the upper respiratory tract is where airborne pathogens initially come into interaction with the body's mucous membranes [31]. Pathogens from surroundings and dust particles are continuously exposed to the pulmonary system and airways. Once there are issues with any component of this system, lung disease can occur [32]. The most common

causes of bacterial lung infections in normal hosts include *Streptococcus pneumoniae, Haemophilus species, Staphylococcus aureus, and Mycobacterium tuberculosis*. The bacterial infections of the lungs were responsible for one-fifth of all fatalities in Europe and North America between the seventeenth and nineteenth centuries. It can remain dormant for years before establishing a chronic cavitating lung infection with highly infectious sputum. Following a significant reduction in prevalence, mainly due to advancements in public health, *M. tuberculosis* infections are currently reducing in rate, while multidrug-resistant strains are spreading across various areas [33, 34]. Other common microorganisms responsible for pneumonia include *Staphylococcus aureus, Group A Streptococcus, Klebsiella pneumoniae, Haemophilus influenzae,* and *Moraxella catarrhalis* [35]*.* Additionally, patients with smoking-related lung disease frequently have *Haemophilus influenzae* infection, which can result in bronchial inflammation and patchy infiltration into the surrounding lung. *H. influenzae* is under-detected by the usual clinical culture approach [36].

#### **2.4 Gut**

Bacteria can enter the intestinal mucosa and replicate there, as well as spread to other organs in the body [37]. The majority of intoxications, including those carried on by *Staphylococcus aureus*, are identified by the symptoms appearing extremely quickly (often within a few hours) [38]. The gut microbiota is a convoluted ecology with approximately 300 to 500 different bacterial species [39]. In contrast to the lower gut, the stomach and upper intestine have less abundant microbiota [40]. Bacteria can be found in the mucosa and in the lumen, although they often are unable to penetrate the gut wall. The usual intestinal microbiota contains small populations of bacteria which can lead to disease when allowed to overgrow. For instance, an over-population of *Clostridium difficile* can result in serious intestinal inflammation and diarrhea. Antibiotic administration starts the procedure by inhibiting the natural microbiota [41]. The main common pathogens of the gut include *Vibrio cholerae,* enteropathogenic strains of *E. coli, Eubacterium, Bacillus cereus, Bacteroides vulgatus, Bifidobacterium, Clostridium difficile, Fusobacterium, Peptostreptococcus, Pseudomonadota, Prevotella, Salmonella enterica, Salmonella gastroenteritis, Salmonella typhimurium,* and *Shigella spp* [42–48]. *Bacillus subtilis* is a gut commensal and non-pathogenic [49, 50]. *Lactobacillus johnsonii* and *Clostridium perfringens* are both commensal [51, 52]. However, *Clostridium perfringens* is also an opportunistic pathogen that can lead to lethal diseases as a result of overgrowth causing gas gangrene, food poisoning, non-foodborne diarrhea, and enterocolitis [53, 54]. *Bacteroides fragilis* is part of the normal microbiota of the human colon and is commensal, but can cause infection if displaced into the bloodstream or surrounding tissue following surgery, disease, wounds, or trauma [55, 56].
